1. Field of the Invention
The present invention relates to hydraulic systems which have multiple pumps connected to a common supply line, and particularly to mechanisms for unloading fluid supplied by one of the pumps when the output of that pump is not required.
2. Description of the Related Art
Numerous types of machines have members which are moved by a hydraulic system. Specifically, a member is driven by an actuator, such as a hydraulic cylinder and piston arrangement, that receives pressurized fluid via a proportional control valve. The control valve is opened varying degrees to proportionally control the rate the fluid flows to or from the associated actuator, thereby moving the machine member at different speeds, as desired by the user.
It is common practice on a tractor loader/backhoe and similar machines to have two fixed displacement hydraulic pumps driven on a common shaft by the machine's engine. In many cases, one of the pumps has its working pressure reduced under certain circumstances in order that the hydraulic horsepower does not exceed the horsepower available from the engine or transmission system. This action, known as “pump unloading”, can be set to occur upon a given event, usually at a certain level of pressure in the main system. Hence this device can be controlled by a relief valve, which directs the pump output flow back to the reservoir at lower pressure.
Hydraulic systems often control the unloading by producing a load-sense signal, which indicates the greatest load applied to the different hydraulic services on the machine. In a system with fixed displacement pumps, the load-sense signal operates a variable relief valve or bypass compensator that opens flow path from the pumps to system reservoir. This single compensator valve maintains the combined pump pressure a fixed amount above the load-sense pressure as determined by a spring force acting on that valve. This pressure difference is often referred to as the “margin.” If the flow required at the service ports of the control valves is greater than or equal to the combined capacity of the pumps, then the unloading path from the pump to tank is closed off. At this point the margin decays below the level set by the spring and is dependent upon the size of the opening which is presented to the downstream pump.
The present inventor has recognized that for optimal engine power savings, it is desirable to provide independent unloading for each pump in a dual pump system and have one pump be subordinate to the other.